1. Field of the Invention
This invention relates generally to a powertrain for a hybrid electric vehicle (HEV), and, in particular to a method for avoiding shift hunting among gears of an automatic transmission.
2. Description of the Prior Art
A motor vehicle powertrain having an internal combustion engine and an automatic transmission, in which discrete gear ratios are produced, typically has a shift schedule calibrated to schedule downshifts when the engine runs out of reserve power and to schedule upshifts as close as possible to tractive effort crossovers. The shift schedule is calibrated for use in a vehicle that is unloaded and on level ground since the vehicle generally operates in these conditions.
When the vehicle is on a significant positive slope, the level ground calibration can cause a condition described generally referred to a shift hunting, wherein the transmission frequently upshifts and downshifts between gears when the driver is trying to maintain vehicle speed on a grade having positive slope. The transmission hunts between upshifts and downshifts because the driver is unable to find an accelerator pedal range that will maintain a desired level of vehicle speed. This condition can occur on a grade having positive slope where the driver is trying to maintain a constant speed.
Shift hunting occurs when the vehicle begins to decelerate on the grade due to the increased road load, causing the driver to tip-in to the accelerator pedal to maintain vehicle speed. As a result of the tip-in, the transmission downshifts. Because the vehicle begins to accelerate beyond the desired speed after the downshift is completed, the driver tips-out of the throttle causing an upshift to occur.
This cyclic shift hunting continues until the slope levels or the vehicle leaves the grade. Shift hunting is undesirable due to the excessive upshifting and downshifting, which cause inability to maintain a desired constant speed on the grade.
If a downshift is executed and a subsequent upshift is prevented, shift hunting can be prevented but the driver is dissatisfied because the transmission is perceived to be unresponsive. Inhibiting the upshift causes a reduction in fuel economy due to remaining in the lower gear.